ABSTRACT

The Jurassic-Early Cretaceous Vizcaíno and Alisitos terranes are juxtaposed in the central part of the Baja California Peninsula, México. The southernmost outcrops of the Alisitos terrane are located in the Sierra El Arco, which consists of greenschist-facies meta-igneous rocks. Further east in the Calmallí area, amphibolite and paragneiss dominate. The meta-igneous rocks constitute a magmatic system ranging from peridotite to granodiorite, with associated basaltic to dacitic volcanic rocks. The suite displays negative εSr_t (average −8.4, n = 23) and positive εNd_t values (average +5.8, n = 23), suggesting mantle-derived melts with no discernible assimilation of continental crustal material. Geochemical and isotopic data of the amphibolite suggests a protolith similar to the meta-igneous rocks. Paragneiss samples display εSr_t of +166 and εNd_t around −7, with Nd crustal residence ages of 1.5 Ga. The isotopic data of the paragneiss suggest provenance from continental sources. Similar to Triassic-Middle Jurassic metasedimentary rocks from the Baja California Peninsula and western México. Middle Jurassic magmatism on Cedros Island (Vizcaíno terrane) and the Sierra El Arco (Alisitos terrane) define an εSr_t vs. εNd_t evolutionary trend that is mostly within the mantle array, suggesting a common origin within the same island arc subduction setting. Jurassic magmatism of continental affinity, defined by positive εSr_t (average +39, n = 15) and negative εNd_t values (average −4.2, n = 15), is widespread in the Baja California Peninsula and western México. Our Sr-Nd isotope mixing curves suggest that the assimilation of Late Triassic metasedimentary rocks or a lower crust could explain the isotopic composition of the continental affinity Jurassic magmatism.

摘要

侏罗纪-早白垩世 Vizcaíno 和 Alisitos 地体并列在墨西哥下加利福尼亚半岛的中部。Alisitos 地体的最南端露头位于 Sierra El Arco，由绿片岩相变质火成岩组成。再往东，在 Calmallí 地区，角闪岩和平行长麻岩占主导地位。变质火成岩构成了一个岩浆系统，范围从橄榄岩到花岗闪长岩，伴生玄武岩到英安质火山岩。该套件显示负εSr_吨（平均-8.4中，n = 23）和正的εNd_吨值（平均 +5.8，n = 23），表明地幔衍生的熔体与大陆地壳物质没有明显的同化作用。角闪岩的地球化学和同位素数据表明其原岩类似于变质火成岩。副片麻岩样品显示 εSr _t为 +166，εNd _t约为 -7，Nd 地壳居住年龄为 1.5 Ga。副片麻岩的同位素数据表明来自大陆来源。类似于来自下加利福尼亚半岛和墨西哥西部的三叠纪-中侏罗纪变沉积岩。塞德罗斯岛 (Vizcaíno terrane) 和 Sierra El Arco (Alisitos terrane) 上侏罗纪中期岩浆活动定义了 εSr _t与 εNd _t演化趋势主要在地幔阵列内，表明在相同的岛弧俯冲环境中有一个共同的起源。大陆亲和力的侏罗纪岩浆，由正εSr定义_吨（平均39中，n = 15）和阴性的εNd_吨值（平均值-4.2中，n = 15），是在下加利福尼亚半岛和墨西哥西部普遍。我们的 Sr-Nd 同位素混合曲线表明晚三叠世变沉积岩或下地壳的同化作用可以解释大陆亲合侏罗纪岩浆作用的同位素组成。